Optical cross connect switches (OXC) are well known to optically switch signals between an array of input fibers to an array of output fibers. Of consideration are OXC's employing 3D free space beams and 2D MEMS mirror arrays. OXCs are also known as an Optical Cross connect Switch OCS, and with the addition of splitters and combiners as a Reconfigurable Add Drop Multiplexer (ROADM).
FIG. 1 is a section of a ray trace of a Prior Art Z bend OXC 100. An input block 110 has a fiber block 111 to arrange input fibers into a 2D array, and a 2D array of collimating lenses 112 couples the input beams from fiber into free space. The pitch is constant, and fiber is centered on the lens. This results in an 2D array of parallel beams 171a-c, 1 per fiber, in free space. Only 2 marginal ports are shown for illustrative purposes. Arrays of 320 fibers are common.
An output block 140 is the same as the input block 110, but with the beams 173a-c travelling in the opposite direction. A 2D array of focusing lenses 142 couples the output beams from 3D free space into a 2D array of fibers 141.
A pair of 2D steering mirror arrays 120 and 130 are arrays of independent flat mirror elements, located between input fiber block 110 and output fiber block 140. Mirror arrays 120 and 130 are parallel to each other. The incidence angle 161 between input block 110 and mirror array 120 is the same as the incidence angle between mirror array 130 and output block 140. Each fiber in the input block 110 to corresponds to a mirror in the input mirror array 120, and each mirror in the output mirror array 130 corresponds to a fiber in the output block 140. The mirrors are arranged in a Z bend configuration, as to steer each beam from an input mirror to an output mirror. For example, beam 171a may be steered to 172a and 173a, or 172c and 173c. By independently adjusting the azimuth and elevation actuation angles 162a-c of each mirror element, the beams are precisely positioned. The angle of the output mirror element is the complement of the angle of the corresponding input mirror. Actuation angle 162a is in the opposite sign of actuation angle 162c, thus the mirror actuator must be capable of twice the angle 162a. 
A controller 150 determines and optimizes the position of each mirror element.
The free space beams are Gaussian. The beam profile suffers from divergence with distance. The point along the beam profile with the smallest beam diameter is the waist.
For the illustrated example, with a 30 degree incidence angle 161 and a +/−20 degree maximum mirror actuation angle 162, the difference in path length 171a to 172a to 173a versus 171a to 172c to 173c is 37.5%. The variation in optical path length results in the waist not being located at the focusing mirror for all mirror angles. Variation in beam size results in a variation of insertion loss, dependent on which switch ports are selected. Optical link budgets are based on worst case insertion loss. This port to port variation in insertion loss can lead to higher insertion loss ports becoming unusable, such as in U.S. Pat. No. 9,210,487 “Implementation Of A Large-Scale Multi-Stage Non-Blocking Optical Switch”.
Also, OXCs are 10 limited, the number of ports is limited by electrical 10 count. Each mirror element actuator requires 4 analog control leads plus spares and grounds. A 320 port switch mirror array package may have over 1900 pins.